1. Field of the Invention
This invention relates to optical glass of high refractive index and los dispersion, and more particularly to optical glass having a refractive index nd in the range of 1.71 to 1.85 and Abbe number .nu.d ranging from 40 to 57.
2. Description of the Prior Art
Optical glass of high refractive index and low dispersion has heretofore been made to contain thorium oxide for imparting the properties of high refractive index and low dispersion. However, the use of thorium oxide as an ingredient is undesirable because thorium is radioactive and harmful to the human body.
Instead of thorium oxide, it has been proposed to use yttrium oxide for imparting the properties of high refractive index and low dispersion to glass. For example, glass constituting a three-ingredient system B.sub.2 O.sub.3 -La.sub.2 O.sub.3 -Y.sub.2 O.sub.3 exhibits a high refractive index and low dispersion. However, the use of these three ingredients alone suffers from a very strong tendency to devitrify, and accordingly, the glass forming region is narrow, thus making it impossible to obtain a stable optical glass having the desired optical constants. An ingredient such as Ta.sub.2 O.sub.5, ZrO.sub.2 or CdO which imparts a high refractive index may be added to the three-ingredient system B.sub.2 O.sub.3 -La.sub.2 O.sub.3 -Y.sub.2 O.sub.3, but if the content of such additional ingredient exceeds 5%, the Abbe number will be decreased, and therefore, the amount of such ingredient must be restricted to a very small percentage, and as a result, glass having a wide range of refractive index cannot be obtained.